1. Field of the Invention
This invention generally concerns rectangular exhaust nozzles for turbojet aircraft engines and particularly concerns a compact and lightweight transition casing that interconnects a rectangular exhaust nozzle to a cylindrical engine casing.
2. Description of Prior Developments
The maneuverability of modern high performance turbojet aircraft is significantly enhanced by extending the capability of the engine exhaust nozzle beyond its conventional jet accelerating function to include a jet deflection capability. Jet deflection in the engine exhaust nozzle can produce more rapid aircraft maneuvers at lower flight speeds than can be achieved by conventional control surfaces. In addition, a reverse thrust capability incorporated with the exhaust nozzle can enable an aircraft to quickly decelerate on landing thereby reducing the landing roll distance for accommodating short landing field operation.
Exhaust nozzles capable of carrying out such additional functions are known as multifunction exhaust nozzles. A typical example of a conventional multifunction exhaust nozzle is schematically represented in FIGS. 1 through 6. Exhaust nozzles of the type shown in FIG. 1 which include a substantially rectangular cross section are often called two dimensional (2D) nozzles. These nozzles are preferred for multifunctional applications, since unlike round section, axisymmetric nozzles, the hinged flaps (10) may be differentially actuated as shown in FIG. 2, thereby deflecting the exhaust gas for rapid pitch maneuvering of the aircraft.
Exhaust nozzles for thrust augmented engines rely on the pivoting or rotation of hinged flaps (10) to selectively vary the cross sectional area of the nozzle as required. The 2D nozzles include four wide, substantially flat surfaces provided by two or more movable flaps (10) and two fixed sidewalls (12), as compared to 12 or more narrow flaps used on conventional axisymmetric nozzles. Even though the total flap surface area is similar for each type of nozzle, 2D nozzles tend to weigh more than axisymmetric nozzles.
During normal cruising conditions, the flaps 10 are oriented symmetrically as seen in FIG. 3. Unlike axisymmetric nozzles, the flaps (10) operating between the two fixed sidewalls (12) may be closed, as shown in FIG. 4. This blocks the flow of exhaust gas so that the gas is discharged through auxiliary exhaust nozzles (14) directed in a forward direction to produce reverse thrust.
While the performance benefits of 2D nozzles are significant, 2D nozzles have consistently weighed more than conventional axisymmetric nozzles. Because of this inherent drawback in weight, the full performance potentional of 2D nozzles has not heretofore been realized. Reducing the weight differential between axisymmetric (circular section) and 2D nozzles is therefore most desirable for improved aircraft propulsion design.
A major factor in the excess weight of a 2D nozzle over an axisymmetric nozzle is the weight of the 2D nozzle transition casing. The transition casing, which is circular in section at its forward end, uniformly changes to a rectangular section at its aft end. In light of this change in flow path configuration, sections through the transition casing of a 2D nozzle are generally non-circular and thus, the internal gas pressure loading cannot be reacted in simple, efficient tension loading as can an axisymmetric nozzle casing.
In contrast to a rectangular casing, a cylindrical casing, as used with an axisymmetric nozzle, can be of very light construction with few, if any, ribs due to its uncomplicated symmetrical load distribution. The transition casing of a 2D nozzle, by comparison, requires numerous ribs supporting a relatively thicker duct skin as necessary to react the panel bending moments resulting from the non-circular casing contour. The difference in the structural handling of internal pressure forces, i.e. simple hoop tension in the case of a cylindrical casing compared to flexure loading within a 2D transition casing, is the basic cause of the large weight difference between axisymmetric and 2D nozzles.
Another drawback associated with 2D nozzles is their inherent aerodynamic incompatibility with single engine aircraft applications. Although the characteristic rectangular cross section of typical 2D nozzles can be smoothly integrated into the contours of twin engine aircraft, in the case of single engine aircraft having generally circular fuselage cross sections, the two dimensional nozzle with its rectangular cross section cannot be readily blended with the arcuate fuselage contours to produce a low drag afterbody. In view of the flight maneuverablity and other benefits of 2D nozzles, a need exists for an improved lightweight transition casing design which is generally compatible with single engine aircraft contours.